The invention relates to a method for testing the serviceability of two-wire transducers in automation engineering that output a measurement signal as an injected current.
The transducer in this case is located remotely in the field environment, local to the process, and is equipped with sensor means for detecting a physical process variable and means for converting it into an electrical variable.
For a multiplicity of transducers, means are kept in a central control area for supplying them with power and for measured-value processing and visualization in the control area.
In a two-wire transducer, both the supply of electrical power to the active components of the transducer and the transmission of the measured values to the means for measured-value processing and visualization in the control area are implemented via a single two-wire line between the transducer in the field environment and a transducer power supply unit located in the control area.
For this purpose, a current loop starting from a voltage source arranged in the transducer power supply unit is formed via the first wire of the two-wire line, the internal resistance of the transducer, the second wire of the two-wire line, and a measuring resistor arranged in the transducer power supply unit, with the loop current of said loop being set by appropriate variation of the internal resistance of the transducer depending on the detected process variable. The voltage drop across the measuring resistor is then a measure of the detected process variable of the transducer.
A minimum input voltage is required across the internal resistance of the transducer to supply the active components of the transducer with power. If the line impedance is too large and/or the output voltage from the transducer power supply unit is too low, a high loop current, whose value depends on the detected process variable, results in a high voltage drop across the two-wire line, with the required minimum input voltage of the transducer not reached in the steady state.
It is particularly problematic that this effect normally remains initially undiscovered when the transducer is put into service, and does not appear until high loop currents occur in the long term.